My older 2 are 5 and 4 and while we work on responding when people talk to them, I will in no way force them to talk to strangers, younger or older who approach them, or act as though they have done something wrong if they don't want to respond to strangers.

They get A LOT of extra attention because they have pretty striking red hair, and it's unusual I guess to see 3 kids with red hair, so they are stopped several times in stores and talked to by strangers wherever we go. It's uncomfortable for me (even though I do respond politely every time), I can only imagine how difficult it is for them as very shy kids to be approached and expected to speak to others they don't know so often.

By 7-10 though, I would expect them to at least acknowledge someone though, even if they don't feel like engaging.